Portable or laptop computers are commonly provided with rechargeable batteries which power the computer when conventional power outlets are not available. An AC adapter is also typically provided to power the computer when the user does have access to an outlet. The AC adapter further provides power to a battery charging power supply which recharges the batteries.
In most portable computer systems, the power supply supplies two levels of battery charging current: one level when the computer system is off and a second when the computer system is on. To ensure that there is always sufficient current available for charging the battery when the computer system is also drawing current, the level of charging current output by the battery charger must be selected in accordance with the maximum current which could be drawn by the system. Thus, the AC adapter must be designed for the situation in which the computer is using maximum power and the battery requires charging. Because the adapter is designed for this "worst case" situation, the size of the AC adapter is often larger than necessary for typical operation where the computer is not drawing the maximum current and the balance of power available from the AC adapter is wasted.
Alternatively, some systems utilize a constant current AC adapter to charge the battery and power the computer. The output of the AC adapter is directly connected to the battery and to the DC/DC power supply which powers the computer system. In this manner, the battery charging power supply is eliminated and all of the current from the adapter that is not used by the computer system is available to charge the battery. However, in this configuration, the output voltage of the adapter is clamped to the battery voltage and the output capabilities of the adapter are significantly reduced. For example, if the AC adapter was rated at 20 Volts with a constant current of 1 amp, the output power of the adapter would be 20 Watts. If a 10 Volt battery was connected to the output of the adapter, the output capacity of the adapter would be reduced by half to 10 Watts.